Ah... looks like the render without IDL also has those poly edge artifacts...

Is this an Irradiance Cache issue?

Lowered the Min Shading Rate in render settings from my stock 0.4, down to 0.2 for that IDL render above, and still got the artifacts.

There's a shadow blur of 2.0 and shading rate of 0.2 set on the single, ray-traced, infinite light, that's providing illumination, alongside the IDL.